1. Field of Invention
The present invention relates to a field of nonaqueous electrolytic solutions and a secondary battery using the same. More particularly, this invention pertains to nonaqueous electrolytic solutions that comprise (a) one or more solvents and (b) two or more ionic salts. The present invention pertains to secondary batteries comprising such nonaqueous electrolytic solutions, and particularly to methods of making nonaqueous electrolytic solutions with at least two salts for use in lithium and lithium ion rechargeable batteries.
2. Description of Related Art
Electrolytic solutions in state-of-the-art lithium ion rechargeable batteries contain ethylene carbonate (EC) as a co-solvent, and lithium hexafluorophosphate (LiPF6) as an electrolytic salt. In the battery system, EC must be used in order to form stable solid electrolyte interface (SEI) that is critical to the cell performance.
LiPF6 has been used as an electrolytic salt due to its good overall properties, although it does not have the best individual properties such as ion conductivity, ion mobility, thermal stability, and electrochemical stability, when compared with other lithium salts such as lithium perchlorate (LiCIO4), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), etc. However, due to its thermal instability at relatively high temperatures (see Reaction 1 below), LiPF6-based electrolytic solutions cannot be used at temperatures above 50° C., which limits the cell performance of lithium ion rechargeable batteries containing LiPF6-based electrolytes in higher temperature applications.LiPF6→LiF+PF5  (1)
On the other hand, LiPF6 is not chemically stable and is easily to decompose by hydrolysis in the presence of residual moisture or acidic impurities in the lithium salt and solvents (see Reaction 2 below).LiPF6+H2O→2HF+LiF+POF3  (2)
The presence of the strong Lewis acid PF5 and strong acid HF in the electrolytic solutions is very harmful to batteries because they react with solvent components and electrode active materials and corrodes the SEI, therefore, resulting in the poor long cycle life performance of the batteries. Hence, there is room for improvement in the selection of an electrolyte for use in secondary batteries.